Carrier 37HS specifications Legend For, Ð Piping Diagram, Heating/Cooling, Sequence, DA Thermostat

System-Powered Controls with Pneumatic Interface

PNEUMATIC SEQUENCED HEATING/COOLING (HOT WATER) Ð By using a 20 psi pneumatic source, one ther- mostat can control both heating and Moduline cooling in the conditioned space. The pneumatic circuit is interfaced with the system-powered circuit by use of a pilot valve. In cool- ing, the Moduline air¯ow is controlled by the duct-powered volume controller just as in all system-powered control. As the schematic diagram shows, a DA pneumatic wall ther- mostat working through the pilot valve, replacing the system- powered thermostat, modulates the pilot valve and thus the volume controller low side pressure.

Figure 75 shows the piping diagram. Figure 76 shows the sequence diagram for a direct acting thermostat with a nor- mally open pilot valve. With the system shut down and the space temperature low, at start-up the thermostat branch pres- sure is low, and the normally open water valve (or other heat- ing means) is open. As the space temperature rises, the branch pressure rises, modulating the water valve to a closed po- sition. The pilot valve is normally open, with set point above water valve spring rate, thus maintaining a low pressure in the low side of the volume controller, closing the bleed valve in the controller and raising the bellows pressure to the duct pressure level; this results in unit shutoff.

If the space temperature continues to rise, the NO pilot valve will modulate toward closed, raising the low side pres- sure, opening the bleed, reducing the bellows pressure, and opening the Moduline unit air valve. The air¯ow will rise until the volume controller setting is reached.

Note that the spring rates of the hot water valve and the pilot valve must be properly matched to accomplish the ac- tion described above.

A heating/cooling sequence can also be accomplished with a reverse acting circuit (Fig. 77 and 78). At start-up, the tem- perature is low, the branch pressure is high, and normally closed water valve is wide open. As the space temperature rises, the branch pressure falls and the water valve closes, stopping the heating supply. The pilot valve is NC and thus at branch pressures above its setting will open, volume con- troller low pressure will be reduced, the bellows pressure will rise and the unit will shut off.

LEGEND FOR FIG. 75 - 78

DA	Ð Direct Acting	NC Ð Normally Closed
HW	Ð Hot Water	NO Ð Normally Open
M	Ð Main	RA Ð Reverse Acting

Fig. 75 Ð Piping Diagram, Heating/Cooling

Sequence, DA Thermostat

Fig. 76 Ð Sequence Diagram Ð DA Thermostat

Fig. 77 Ð Piping Diagram, Heating/Cooling

Sequence, RA Thermostat

Fig. 78 Ð Sequence Diagram, RA Thermostat

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